1. Field of the Invention
The present invention relates to an engine starting system for a motor vehicle and particularly to an engine starting system capable of supplying large current to a starter without burdening s battery with electrical loads when an engine is started.
2. Prior Art
FIG. 1 shows a construction of a representative engine starting system using a starter. Generally, in the engine starting system, the torque generated in the starter rotates a crank shaft through a pinion for starting the engine.
The starter assembly 10 comprises a magnet section 12 and a starter 14. A battery 16 is connected with a solenoid coil 22 of the magnet section 12 through a fusible link 18 and a starter switch 20. Further, the battery 16 is connected with a terminal 24a of a main switch 24 for supplying current to the starter 14. The starter 14 is a series-wound direct current type motor comprising a pinion 26 disposed on a thrust shaft of an armature 32, an exciting coil 28, an armature coil 30 and a commutator 34.
When the starter switch 20 is turned on, current is supplied from the battery 16 to the solenoid coil 22 and the plunger 36 is shifted in the direction indicated by an arrow 100 to turn the main switch 24 of the magnet section 12 on. Thus, current from the battery 16 is supplied to the starter 14 through a main line 200. At this moment, a shift fork 38 is rotated to move the pinion 26 in the direction indicated by an arrow 300, thereby the pinion 26 is meshed with a ring gear 40. Thus, the torque of the starter 14 is transferred to the ring gear 40 to rotate the crank shaft for starting the engine.
FIG. 2 presents miscellaneous characteristics of the starter 14 with respect to the current flowing therethrough. In this drawing, a curve (a) indicates output characteristic, a curve (b) does rotational speed characteristics, a curve (c) does torque characteristics and a curve (d) does voltage characteristics. For example, when the rotational speed of the starter 14 is zero, both current and torque become maximum. Torque is reduced with an increase of the rotational speed. Torque is proportional to the square of current. In a row current area the square characteristic is outstanding and in a high current area the increase rate of torque becomes constant due to the saturation of lines of magnetic flux.
In order for the starter to produce an output as shown in the specification, the output characteristics of the battery must be larger than that of the starter, that is to say, the curve (e) must always stay above the curve (d).
FIG. 3 shows a change of the starter current Id in the process from the standstill state to the starting state of the engine. The change of the starter current Id is composed of three components, namely, first one is a lock area current component (hereinafter referred to as lock current) 50, second one is an alternating current component 52 in which current pulsates according to the reciprocating motion of the piston and third one is a direct current component which corresponds to friction of the mechanical parts of the engine. Here, the lock current area means a period from when the engine is initially rotated by the starter until when it reaches a rotational number enabling to start the engine.
The direct current component comprises a current component 54 corresponding to static friction to be overcome at the initial rotation of the engine and a current component 56 corresponding to dynamical frictions caused during the rotation of the engine. When the engine is cold, a larger current component corresponding to an increased static friction is added. In FIG. 3, the period from the engine stop to the engine starting is called a lock current area time 70 including a period 58 from the engine stop to the engagement of the pinion with the ring gear and a period 60 of the current mainly corresponding to static frictions.
The discharge characteristic of the battery is lowered due to the life of the battery, cold temperature conditions and the like. In an extreme case, the battery discharge characteristic becomes lower than the voltage characteristics of the starter. As shown in FIG. 3, in order to overcome frictions and rotate the crank shaft, the magnitude of the lock current 50 must be larger than a value determined by the capacity of the battery and the resistance of the starter. Followings are conditions which are generally required in designing the battery and starter system.
(1) The battery capacity capable of supplying a lock current enough to overcome the static frictions at starting the engine.
(2) The life of the battery is affected especially by deep discharge conditions encountered each time of the engine starting.
(3) A voltage drop caused by wiring resistance must be taken into consideration when the lock current is supplied. For reducing the voltage drop, a large diameter of wiring is needed.
For example, Japanese Unexamined Patent Application Toku-Kai-Hei 2-175350, Unexamined Utility Model Application Jitsu-Kai-Hei 3-82876 and others disclose techniques to solve these problems. However, these prior arts are still insufficient to solve the problems associated with the lock current.